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Lecture – 3 / cell injury
Dr Hussain Abady
Mechanism of cell injury
The mechanisms responsible for cell injury are complex, however, several principles are
relevant to most forms of cell injury;
1. The cellular response to injurious stimuli depends upon;
a. nature of the injury.
b. duration of injury.
c.
severity of injury.
So mild transient injurious agent may induce reversible cell injury, whereas severe one
results in irreversible cell injury or instantaneous cell death.
2. The consequences of cell injury depend upon;
a. type of injured cells.
b. state of injured cells.
c. adaptability of the injured cell.
d. genetic makeup of injured cells.
3. Cell injury is result from different biochemical mechanisms acting on several
essential cellular components and simultaneously trigger multiple
interconnected mechanisms that damage cells.
4. Cellular components that are most frequently damaged by injurious stimuli
include;
a. Mitochondria.
b. Membranes.
c. Machinery of protein synthesis and packaging.
d. DNA in nuclei.
5. The principal mechanisms of cell injury as in figure below and include;
1. ATP depletion
ATP depletion and decreased ATP synthesis is frequently the result of both hypoxic and
chemical (toxic) injury. ATP is produced in two ways;
a. The major pathway is oxidative phosphorylation of adenosine diphosphate in
mitochondria in presence of oxygen.
b. The second is the glycolytic pathway, which can generate ATP in the absence of
oxygen using glucose.
The major causes of ATP depletion are;
i.
reduced supply of oxygen and nutrients,
ii.
mitochondrial damage,
iii.
actions of some toxins (e.g., cyanide which block oxidative phosphorylation).
Tissues with a greater glycolytic capacity (e.g., the liver) are able to survive loss of
oxygen and decreased oxidative phosphorylation better than are tissues with limited
capacity for glycolysis (e.g., the brain).
Depletion of ATP to 5% to 10% of normal levels has widespread effects on many critical
cellular systems:
a) Reduction of activity of the plasma membrane energy-dependent sodium pump
(ouabain-sensitive Na+, K+-ATPase).
b) Cellular energy metabolism is altered. decrease in cellular ATP stimulate, leading to
an increased rate of anaerobic glycolysis. As a consequence, glycogen stores are rapidly
depleted with accumulation of lactic acid. This reduces the intracellular pH (acidosis).
c) Failure of the Ca2+ pump leads to influx of Ca2+, with damaging effects on
numerous cellular components (high intracellular Ca+ stimulates enzymes
phospholipase, endonucleases, and ATPase).
d) With prolonged or worsening depletion of ATP, structural disruption of the protein
synthetic apparatus occurs, manifested as detachment of ribosomes from the rough ER
and dissociation of polysomes, with a consequent reduction in protein synthesis.
proteins may become misfolded, and misfolded proteins trigger a cellular reaction called
the unfolded protein response that may culminate in cell injury and even death by
apoptosis.
2. Mitochondrial damage
Mitochondria are the cell's suppliers of energy in the form of ATP. Mitochondria is
sensitive to virtually all types of injurious stimuli can be damaged by;
a. increases of cytosolic Ca2+.
b. reactive oxygen species.
c. oxygen deprivation.
d. toxins.
e. mutations in mitochondrial genes cause of some inherited diseases.
There are two major consequences of mitochondrial damage;
a) Mitochondrial damage often results in failure of oxidative phosphorylation and
progressive depletion of ATP, culminating in necrosis of the cell.
b) The mitochondria also sequester between their outer and inner membranes several
proteins that are capable of activating apoptotic pathways; these include cytochrome c
and proteins that indirectly activate apoptosis inducing enzymes called caspases.
3. INFLUX OF CALCIUM AND LOSS OF CALCIUM HOMEOSTASIS;
Ischemia and certain toxins cause an increase in cytosolic calcium concentration.
Increased intracellular Ca2+ causes cell injury by several mechanisms;
I.
The accumulation of Ca2+ in mitochondria results in opening of the
mitochondrial permeability transition pore and failure of ATP generation.
II.
Increased cytosolic Ca2+ activates a number of enzymes;
i.
phospholipases (which cause membranes damage),
ii.
proteases (which break down both membrane and cytoskeletal proteins).
iii.
endonucleases (which are responsible for DNA and chromatin fragmentation).
iv.
ATPases (thereby hastening ATP depletion).
III.
Increased intracellular Ca2+ levels also result in the induction of apoptosis, by
direct activation of caspases and by increasing mitochondrial permeability.
4. ACCUMULATION OF OXYGEN-DERIVED FREE RADICALS
(OXIDATIVE STRESS);
Cell injury induced by free radicals, particularly reactive oxygen species, is
an important mechanism of cell damage in many pathologic conditions,
such as
a. Chemical injury.
b. radiation injury.
c. ischemia-reperfusion injury.
d. cellular aging.
e. microbial killing by phagocytes.
Free radicals are chemical species that have a single unpaired electron in an outer orbit
and characterized by;
a) Free radicals react with inorganic or organic chemicals—proteins, lipids,
carbohydrates, nucleic acids—many of which are key components of cell
membranes and nuclei.
b) Moreover, free radicals initiate autocatalytic reactions, whereby molecules
with which they react are themselves converted into free radicals, thus
propagating chain of damage.
I. Reactive oxygen species (ROS); are a type of oxygen-derived free radical that are
produced normally in cells during mitochondrial respiration and energy generation, but
they are degraded and removed by cellular defense systems.
Oxidative stress; Occurs when the production of ROS increases or the scavenging
systems are ineffective, the result is an excess of these free radicals. Oxidative stress has
been implicated in a wide variety of pathologic processes, including;
a) cell injury.
b) Cancer.
c) Aging.
d) some degenerative diseases such as Alzheimer disease.
e) ROS are also produced in large amounts by leukocytes, particularly neutrophils
and macrophages in inflammatory reactions as mediators for destroying
microbes, dead tissue, and other unwanted substances.
Types of ROS are:
i.
Superoxide (O2-) are produced from oxygen by oxidative enzymes in the
endoplasmic reticulum, mitochondria, plasma membrane, peroxisomes, and
cytosol.
ii.
Hydrogen peroxide (H2O2), is produced from (O2-) by dismutation by action of
superoxide dismutase (SOD). H2O2 is also derived directly from oxidases in
peroxisomes.
iii.
Hydroxyl radical (OH-) is produced from H2O2 by the Cu2+/Fe2+ by Fenton
reaction. The absorption of radiant energy (e.g., ultraviolet light, x-rays). Ionizing
radiation can hydrolyze water into hydroxyl (OH-) and hydrogen (H-) free radicals.
Mechanisms for removal of free radicals; Free radicals are inherently unstable and
decay spontaneously. There are also several non-enzymatic and enzymatic systems that
contribute to inactivation of free-radical reactions;
A. Superoxide dismutase (SOD) in mitochondria coverts superoxide (O2-) into H2O2.
B. Glutathione peroxidase in mitochondria converts hydroxyl radical (OH-) into
hydrogen peroxide H2O2.
C. Catalase in peroxisomes coverts hydrogen peroxide H2O2 into H2O and oxygen
(O2).
D. Endogenous or exogenous antioxidants (e.g., vitamins E, A, and C, green tea and
β-carotene) may either block the formation of free radicals or scavenge them once
they have formed.
E. Iron and copper can catalyze the formation of ROS. The levels of these reactive
metals are reduced by binding of the ions to storage and transport proteins (e.g.,
transferrin, ferritin, lactoferrin, and ceruloplasmin).
Pathologic effect of ROS
ROS react with following structures causing cell injury and death;
1) React with fatty acids causing oxidation with generation of lipid peroxidases
leading to disruption of plasma membrane and organelles membranes.
2) React with proteins causing oxidation leading to loss of enzymatic activity and
abnormal protein folding.
3) React with DNA causing oxidation resulting in mutations and breaks.
II.
III.
Enzymatic metabolism of exogenous chemicals or drugs can generate free
radicals that are not ROS but have similar effects (e.g., CCl4 can generate CCl3,
described later in the chapter).
Transition metals such as iron and copper donate or accept free electrons during
intracellular reactions and catalyze free radical formation, as in the Fenton
reaction (H2O2 + Fe2+ ➙ Fe3+ + OH + OH-).
IV.
Nitric oxide (NO), an important chemical mediator generated by endothelial
cells, macrophages, neurons, and other cell types, can act as a free radical.
5. Defects in Membrane Permeability
Early loss of selective membrane permeability leading membrane damage is a consistent
feature of most forms of cell injury (except apoptosis).
Biochemical mechanisms that contribute to membrane damage are;
a. Decreased phospholipid synthesis; due to fall in ATP levels, leading to decreased
energy-dependent enzymatic activities.
b. Increased phospholipid breakdown; due to activation of endogenous
phospholipases by increased levels of cytosolic Ca2+. Oxygen free radicals cause
injury to cell membranes by lipid peroxidation.
c. Cytoskeletal abnormalities; Cytoskeletal filaments serve as anchors connecting
the plasma membrane to the cell interior. Activation of proteases by increased
cytosolic Ca2+ may cause damage to elements of the cytoskeleton.
d. Lipid breakdown products; These include un-esterified free fatty acids, acyl
carnitine, and lysophospholipids result of phospholipid degradation by their
detergent effect on membranes and by causing changes in permeability and
electrophysiologic alterations.
Causes membrane damage are;
a) ischemia,
b) various microbial toxins,
c) lytic complement components,
d) variety of physical and chemical agents.
The most important sites of membrane damage during cell injury are;
i.
Mitochondrial membrane damage; damage to mitochondrial membranes
results in decreased production of ATP, culminating in necrosis, and release of
proteins that trigger apoptotic death.
ii.
Plasma membrane damage; Plasma membrane damage leads to loss of
osmotic balance as well as loss of cellular contents.
iii.
lysosomal membranes; results in leakage of their enzymes into the cytoplasm.
Lysosomes contain RNases, DNases, proteases, glucosidases, and other enzymes
leading to enzymatic digestion of cell components, and the cells die by necrosis.
6. Damage to DNA and Proteins
Cells have mechanisms that repair damage to DNA. P53 (tumor suppressor gene) stops
cell cycle of DNA damaged cells and induce DNA repair by DNA repair enzymes, but if
this damage is too severe to be corrected, P53 will induces cell death by apoptosis
(suicide program). Improperly folded proteins, whether due to inherited mutations or to
injury such as by free radicals also trigger apoptosis.
Examples of cell injury and necrosis
1. Ischemic and hypoxic injury;
Ischemia (diminished blood flow to a tissue) resulting in diminished supply of
oxygen and nutrients to tissues. It is the most common cause of cell injury in clinical
medicine, while hypoxia is reduced oxygen delivery to tissues. Ischemia injures
tissues faster than does hypoxia. Ischemia and hypoxia results in reduced ATP
production with failure of energy dependent essential systems in the cells, if continue
results in irreversible cell injury and death by necrosis.
2. Ischemia-reperfusion injury;
Usually restoration of blood flow to reversibly injured cells can result in cell
recovery. However, under certain circumstances it results, paradoxically, in
exacerbated and accelerated injury especially in myocardial and cerebral infarctions.
3. Chemical (toxic) injury;
Some chemicals act directly by combining with a critical molecular component or
cellular organelle, (e.g.) in mercuric chloride poisoning, mercury binds to the
sulfhydryl groups of various cell membrane proteins, causing inhibition of ATPdependent transport and increased membrane permeability. Metabolites of other
drugs and chemicals such as Carbon tetrachloride (CCl4) and the analgesic
acetaminophen cause cell injury directly or by free radical’s formation.
Serum markers for cell necrosis
Leakage of intracellular proteins through the damaged cell membrane and ultimately
into the circulation provides a means of detecting tissue-specific cellular injury and
necrosis using blood serum samples for example;
a. Cardiac muscle enzymes; for example, contains a specific isoform of the
enzyme creatine kinase-MB and of the contractile protein troponin, estimation
of these protein help in diagnosis of myocardial infarction
b. Bile duct epithelium contain an isoform of the enzyme alkaline phosphatase;
and hepatocytes contain transaminase enzymes (SGPT & SGOT), estimation
of SGPT & SGOT help in diagnosis of hepatocellular diseases, while
estimation of Alkaline phosphatase helps in diagnosis of biliary diseases.